Valves for controlling large volumes of fluid at high operating pressures are known. Such valves have relied on heavy duty design of generous thicknesses or proportions to withstand or contain the large forces resulting from the high operating pressures. Such design resulted in valves that were both expensive to build and bulky for a particular pipe size. In addition, they were extremely difficult to operate and often required expensive heavy duty actuators for reliable operation.
To avoid some of the problems in operating these valves, the moving parts or operating mechanism exposed to the contained fluid, usually the stem and flow closure elements, were sealed or made pressure responsive in such a manner that the resulting forces were substantially balanced or offset. This enabled valve operation with smaller forces or actuators, but the valve mechanisms were complex having a tendency to malfunction and as a result were more expensive to manufacture. Such a valve, commonly referred to as a choke, is manufactured by the Thornhill-Graver Division of Galveston-Houston Company. In this valve, a replaceable seat ring through which the flow is directed is sealed to the valve housing and the tubular flow closure element is spaced from the seat to enable flow and is reciprocated into sealing engagement with the seat for closing off flow. A tubular guide member provides a movement guide for the reciprocating flow closure element which is also sealed with the guide member to preclude the greater upstream pressure from providing an unbalanced pressure urging on the flow closure element. The flow closure element is also formed with a fluid bypass port for enabling the downstream fluid pressure to provide substantially offset urging on the flow closure element to balance or eliminate any operation or movement resistive force. With this arrangement, the only significant unbalanced force on the operating mechanism is that produced by the sealed area of the valve stem. Since this is a relatively small pressure responsive area the resultant unbalanced force is small and easily overcome by standard valve actuators or operating mechanisms.
To minimize the problem of abrasive matter contained with the fluid eroding or cutting out the valve seat, the valve is fabricated with a hardened seat that is easily replaced. The sealing surfaces of the replaceable seat are not protected from the flow and seat damage with resultant valve leakage occurs rapidly requiring frequent repair and replacement. While this arrangement provided for ease of repair it did not provide a satisfactory length of service between repairs.